This invention relates to a method for manufacturing a microlens substrate wherein a plurality of microlenses are formed and arranged, and also to a method for manufacturing a liquid crystal panel using the microlens substrate.
In recent years, flat displays have shown a pronounced spread and breakthrough on the market, including plasma displays, liquid crystal direct view, large-sized screen displays, field emission displays, and organic and inorganic EL displays. These have gained a public favor as being flat and large in size and having a thin design called “Wall-hung TV”, which is considered as one of factors of making a breakthrough.
On the other hand, extensive developments have been made on projectors using light bulbs such as LCD (liquid crystal display), DMD (digital mirror display), LCOS (liquid crystal on silicon) and the like. Although these cannot be made so thin as “wall-hung type ”as set out hereinabove, rear projection TV is fully responsible for high image quality and low costs and has the ability of playing a role of large-sized displays. Moreover, with projectors, they have such applicability as to be arbitrarily selectable in projection frame size that is an intrinsic characteristic property thereof, like front AV projectors. In addition, they have the feature in that their portability is good as a result of the advance in miniaturization, thus ensuring outdoor use. It is thus believed that in some future day, these displays would begin to be put on the respective fields of market depending on the characteristic feature thereof.
In a high-definition liquid crystal panel (liquid crystal light bulb) for conventional liquid crystal projectors, a microlens substrate wherein a microlens is provided in every pixel is used. For the formation of the microlens substrate, a silica substrate or different types of glass substrates are employed, and application of (a) wet etching technique, (b) 2P (photo-polymerization) technique and (c) dry etching technique has been reduced into practice.
Among these techniques, the wet etching technique (a) is illustrated with reference to FIGS. 7A to 7E. Initially, as shown in FIG. 7A, a resist pattern 102 is formed on a substrate 101 made of glass or silica. Next, as shown in FIG. 7B, a lens-shaped concave curve 101a is formed in the substrate 101 by isotropic etching through the mask of the resist pattern 102 with use of a HF etchant. After removal of the resist pattern 102, a resin 103 is applied onto the substrate 101 to fill the inside of the concave curve 101a with the resin as shown in FIG. 7C. Next, as shown in FIG. 7D, a cover glass 104 is attached through the resin 103 on the substrate 101, thereby forming a microlens 105 wherein the resin 103 is filled within the concave curve 101a. Thereafter, as shown in FIG. 7E, an ITO electrode 106 is formed on the cover glass 104 to complete a microlens substrate 107. It will be noted that for the isotopic etching mask used in FIG. 7B, a material such as a metal (chromium or the like) or an impurity-containing polysilicon, which is resistant to a HF etchant, may be used.
Alternatively, another method may be used wherein a resist pattern on a substrate is thermally processed into a lens form, and the substrate is etched through a mask of the pattern to transfer a lens-shaped convex curve in the substrate (see Japanese Laid-open Patent Laid-open No. 2001-92365, particularly at [0008] to [0009] and FIG. 6).
In both methods illustrated hereinabove, the microlens is formed of a resin, and the source of lens power is based on the difference in refractive index between the resin and the substrate. Especially, in the 2P technique (b), the use of a UV-curable resin is essential therefor, and it is unavoidable to use a UV-curable resin.
The resins used for such a microlens substrate should have the following properties (1) to (6).    (1) High transmittance in a visible light region.    (2) High heat resistance (to a temperature of about 200° C.) standing use in the manufacturing process of a liquid crystal display after the formation of microlenses.    (3) Goods light fastness.    (4) Good chemical resistance, or a resistance to chemicals (alcohols, ketones, and waterproofing) in subsequent processes.    (5) High reliability such as of not causing cloudiness by the influence of high temperatures, high humidity, low temperatures and heat cycles and undergoing little change of refractive index without cracking. Optimum viscosity (of about 100 cps to 500 cps), good adhesion and adhesion strength for ensuring uniformity in thickness of a resin film.
However, only a very small number of resins which actually meet the properties (1) to (6) in practice are known. Hence, it is the usual practice to search for a resin that meets such requirements as set out above and optimally design the shape of microlens according to the refractive index depending on the type of device, with the attendant problem that the selection of material is difficult. Especially, where a resin for microlens is used as an adhesive or an adhesive resin used is of a type different from that of a microlens, the requirements other than the visible light transmittance (1) pose problems to all of actually existing liquid crystal displays.
For instance, the problem involved in improving light fastness (3) is as follows. The improvement in brightness of recent liquid crystal projectors increasingly places importance on the improvement in light fastness of a resin (i.e. an organic material) for arranging a liquid crystal panel. Especially, if light in a blue region (in the vicinity of 400 nm), which is an emission region of a lamp used for the projector, is absorbed only slightly, the resin is liable to be degraded to a non-negligible extent owing to the improvement of brightness. In ordinary projectors, light in a wavelength region of 420 nm or below is cut off by means of a UV-IR cut filter or the like. However, it has now been experienced that a slight variation in performance of UV-IR cut filters permits such a non-negligible degree of resin degradation as to occur by the action of light of the component contained within the above-mentioned wavelength region. In the worst case, the resin will undergo yellowing or browning along with an instance where the resin becomes wavy such as by a deformation stress caused during the use of a projector.
With respect to the chemical resistance (4), alcohols and ketones are used in a LCD assembling process including cleaning steps of substrate and panel. Only slight dissolution in these solvents influences a voltage retention of LCD and contributes to ion conduction, thus bringing about the degradation of LCD. Moreover, if such a resin is formed on an aligned film even in the form of a monomolecular layer, a variation in pretilt angle of liquid crystal molecules, an anchoring characteristic of liquid crystals molecules and working operation are influenced.
Further, with respect to reliability (5), resins are usually not resistant to moisture. Water molecules fundamentally degrade adhesion by diffusion in resin, and the refractive index (n=1.33) of water is generally smaller than refractive indices of resins, so that as a moistureproof test times passes, the resin refractive index changes, thereby causing the power of microlens to be degraded and the focal distance to be changed.
The requirement (6) presents problem particularly on the difference in thermal expansion between a substrate material and a resin. In general, the difference in thermal expansion is on the order of magnitude of double-digit or over.
In general, organic matters are low in refractive index, for which it is necessary to form a lens deeply for the purpose of achieving high brightness. Usually, it is more difficult to make a deep lens. When using etching or the like, uniformity and productivity are adversely influenced, and where a substrate size is increased for cost cutting, a difficulty is involved in the uniformisation of a lens shape.